The Axion Dark Matter Experiment (ADMX, also written as ''Axion Dark Matter eXperiment'' in the project's documentation) uses a resonant microwave cavity within a large superconducting magnet to search for cold dark matter

axions An axion () is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interes ...

in the local galactic

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

halo. Unusual for a dark matter detector, it is not located deep underground. Sited at the Center for Experimental Nuclear Physics and Astrophysics (CENPA) at the

University of Washington The University of Washington (UW, simply Washington, or informally U-Dub) is a public research university in Seattle, Washington. Founded in 1861, Washington is one of the oldest universities on the West Coast; it was established in Seattle a ...

, ADMX is a large collaborative effort with researchers from universities and laboratories around the world.

Background

The

axion An axion () is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interes ...

is a hypothetical elementary particle originally postulated to solve the

strong CP problem The strong CP problem is a puzzling question in particle physics: Why does quantum chromodynamics (QCD) seem to preserve CP-symmetry? In particle physics, CP stands for the combination of charge conjugation symmetry (C) and parity symmetry (P) ...

. The axion is also an extremely attractive

candidate. The axion is the puzzle piece allowing these two mysteries to fit naturally into our understanding of the universe.

Strong CP problem

The axion was originally postulated to exist as part of the solution to the "strong CP problem". This problem arose from the observation that the

strong force The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...

holding nuclei together and the weak force making nuclei decay differ in the amount of

CP violation In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics should be the ...

in their interactions.

Weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...

was expected to feed into the strong interactions (

QCD In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type o ...

), yielding appreciable

CP violation, but no such violation has been observed to very high accuracy. One solution to this Strong CP Problem ends up introducing a new particle called the

. If the axion is very light, it interacts so weakly that it would be nearly impossible to detect but would be an ideal dark matter candidate. The ADMX experiment aims to detect this extraordinarily weakly coupled particle. 1e0657 scale

Dark matter

Although dark matter can't be seen directly, its gravitational interactions with familiar matter leave unmistakable evidence for its existence. The universe we see today simply wouldn't look the way it does without dark matter. Approximately five times more abundant than ordinary matter, the nature of dark matter remains one of the biggest mysteries in physics today. In addition to solving the

, the axion could provide an answer to the question "what is dark matter made of?" The axion is a neutral particle that is extraordinarily weakly interacting and could be produced in the right amount to constitute dark matter. If the dark matter accounting for the bulk of all matter in our universe is axions, ADMX is one of only a few experiments able to detect it.

History

Pierre Sikivie Pierre Sikivie (born 29 October 1949) is an American theoretical physicist and currently the Distinguished Professor of Physics at University of Florida in Gainesville, Florida. He invented the axion haloscope and the axion helioscope and has play ...

invented the axion haloscope in 1983. After smaller scale experiments at the

University of Florida The University of Florida (Florida or UF) is a public land-grant research university in Gainesville, Florida. It is a senior member of the State University System of Florida, traces its origins to 1853, and has operated continuously on its ...

demonstrated the practicality of the axion haloscope, ADMX was constructed at

Lawrence Livermore National Laboratory Lawrence Livermore National Laboratory (LLNL) is a federal research facility in Livermore, California, United States. The lab was originally established as the University of California Radiation Laboratory, Livermore Branch in 1952 in response ...

in 1995. In 2010 ADMX moved to the Center for Experimental Physics and Astrophysics (CENPA) at the

. Led by Dr. Leslie Rosenberg, ADMX is undergoing an upgrade that will allow it to be sensitive to a broad range of plausible dark-matter axion masses and couplings.

Experiment

The experiment (written as "eXperiment" in the project's documentation) is designed to detect the very weak conversion of dark matter axions into microwave photons in the presence of a strong magnetic field. If the hypothesis is correct, an apparatus consisting of an 8 tesla magnet and a cryogenically cooled high-Q tunable microwave cavity should stimulate the conversion of axions into photons. When the cavity's resonant frequency is tuned to the axion mass, the interaction between nearby axions in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * ''Halo'' (franch ...

and ADMX's magnetic field is enhanced. This results in the deposit of a very tiny amount of power (less than a yoctowatt) into the cavity. An extraordinarily sensitive microwave receiver allows the very weak axion signal to be extracted from the noise. The experiment receiver features quantum-limited noise performance delivered by an exotic Superconducting QUantum Interference Device (SQUID) amplifier and lower temperatures from a ³He refrigerator. ADMX is the first experiment sensitive to realistic dark-matter axion masses and couplings and the improved detector allows an even more sensitive search. ADMX magnet installation

Cavity

The microwave cavity within the magnet bore is at the heart of ADMX. It is a circular cylinder, 1 meter long and 0.5 meter diameter. ADMX searches for axions by slowly scanning the cavity resonant frequency by adjusting positions of two tuning rods within the cavity. A signal appears when the cavity resonant frequency matches the axion mass. The expected signal from axion decay is so small that the entire experiment is cooled to well below 4.2 kelvin with a liquid helium refrigerator to minimize thermal noise. The electric field within the cavity is sampled by a tiny antenna connected to an ultra-low-noise microwave receiver.

Receiver

The ultra-low noise microwave receiver makes the experiment possible. The dominant background is thermal noise arising from the cavity and the receiver electronics. Signals from the cavity are amplified by an exotic cryogenic Superconducting QUantum Interference Device (SQUID) amplifier followed by ultralow noise cryogenic

HFET A high-electron-mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a heterojunction) ...

amplifiers. The receiver then downconverts microwave cavity frequencies to a lower frequency that can be easily digitized and saved. The receiver chain is sensitive to powers smaller than 0.01 yoctowatts; this is the world's lowest-noise microwave receiver in a production environment.

Progress

ADMX has already (2010) eliminated one of the two axion benchmark models from 1.9 μeV to 3.53 μeV, assuming axions saturate the Milky Way's halo. ADMX hopes to exclude or discover 2 μeV to 20 μeV dark matter axions within the next 10 years. ADMX is undergoing an upgrade to the "Definitive Experiment"; this is sensitive to a very broad range of plausible dark-matter axion masses and couplings. Greater sensitivity will be possible with the upgrade to SQUID amplifiers and the addition of a

dilution refrigerator A 3He/4He dilution refrigerator is a cryogenics, cryogenic device that provides continuous cooling to temperatures as low as 2 Kelvin, mK, with no moving parts in the low-temperature region. The cooling power is provided by the heat o ...

SQUID amplifiers

Several years ago, the ADMX amplifier

noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. (This is to be distinguished from Temperature Noise in Thermodynamics or Principal Interferrometric Analysis Over C ...

was around 2 K. Recently the amplifiers were replaced by SQUID amplifiers, which greatly lowered the noise (to less than 100 mK) and vastly improved sensitivity. ADMX has demonstrated that the SQUID amplifier allows for quantum-limited-power sensitivity. More recently, ADMX has acquired Josephson Parametric Amplifiers which allow quantum noise limited searches at higher frequencies.

Dilution refrigerator

The addition of a dilution refrigerator is the main focus of the ADMX upgrade program. The dilution refrigerator allows cooling the apparatus down to 100 mK or less, reducing the noise to 0.15 K, which makes data taking 400 times faster. This makes it the "Definitive Experiment".

References

External links

ADMX web site
{{Dark matter Experiments for dark matter search